Holographic Data Storage
Developers of information storage devices continue to seek increased storage capacity. As part of this development, memory systems employing holographic optical techniques, referred to as holographic memory systems, holographic storage systems, and holographic data storage systems, have been suggested as alternatives to conventional memory devices.
Holographic memory systems may read/write data to/from a photosensitive storage medium. When storing data, holographic memory system often record the data by storing a hologram of a 2-dimensional (2D) array, commonly referred to as a “page,” where each element of the 2D array represents a single data bit. This type of system is often referred to as a “page-wise” memory system. Holographic memory systems may store the holograms as a pattern of varying refractive index and/or absorption imprinted into the storage medium.
Holographic systems may perform a data write (also referred to as a data record operation, data store operation, or write operation) by combining two coherent light beams, such as laser beams, at a particular location within the storage medium. Specifically, a data-encoded signal beam, also called a data beam, is combined with a reference light beam to create an interference pattern in the photosensitive storage medium. The interference pattern induces material alterations in the storage medium to form a hologram.
Holographically stored data may be retrieved from the holographic memory system by performing a read (or reconstruction) of the stored data. The read operation may be performed by projecting a reconstruction or probe beam into the storage medium at the same angle, wavelength, phase, position, etc., or compensated equivalents thereof, as the reference beam used to record the data. The hologram and the reference beam interact to reconstruct the signal beam.
The reconstructed signal beam (aka a reconstructed data beam) may then be detected by a power-sensitive detector and processed for delivery to an output device. The irradiance impinging on the detector can be written as:I(x,y)=|ES(x,y)+EN(x,y)|2 I(x,y)=|ES|2+|EN|2+2|ES∥EN|cos φS-N where ES(x,y) and EN(x,y) are the scalar complex amplitudes of the holographic signal and the coherent optical noise, respectively. The relative phase difference between the two fields, φS-N, is effectively random, so the cosine factor in the final term swings randomly between +1 and −1. This term, which has the signal multiplied by the noise rather than adding to it, is a limiting noise factor in the practical development of holographic data storage.
Direct detection has several limitations. First, since hologram diffraction efficiency is driven to the lowest possible level in order to maximize the number of pages that may be stored, the read signals may be weak and require long exposure times to detect. Secondly, the laser light used to perform the read-out may be necessarily coherent, thus optical noise sources such as scatter and ISI (intersymbol interference, or pixel-to-pixel crosstalk from blur) may mix coherently with the desired optical signal, reducing signal quality when compared to additive noise of the same power. As such, there may be a need to improve the signal level of the detected hologram and improve the signal to noise ratio (SNR).
Quadrature Homodyne Detection
One way to boost the SNR is to use homodyne detection. In homodyne detection, the reconstructed signal beam interferes with a coherent beam, known as a local oscillator (LO) or LO beam, at the detector to produce an interference pattern that represents a given data page stored in the holographic memory. The detector array produces a signal (e.g., a photocurrent) whose amplitude is proportional to the detected irradiance, which can be written as:Ihomo=|ELO+ES+EN|2 Ihomo=|ELO|2+|ES|2+|EN|2+2|ELO∥ES|cos φLO-S+2|ELO∥EN|cos φLO- N+2|ES∥EN|cos φS-N where ELO is the complex amplitude of the LO. If the amplitude of the LO is much larger than the amplitude of the reconstructed signal beam, then the terms not involving ELO become negligible. This has the effect of amplifying the signal, eliminating nonlinear effects of coherent noise, and allowing the detection of phase as well as amplitude.
To reproduce the data page accurately, however, the LO should be optically phase-locked with the reconstructed data page signal in both time and space such that the LO constructively interferes with each and every data pixel in the hologram simultaneously. However, alignment tolerances, lens aberrations, wavelength and temperature sensitivities, and a host of other minute deviations from perfection may introduce small variations in the flatness of the “phase carrier” wavefront bearing the reconstructed data page. For binary modulation, the “phase carrier” wavefront may be defined as the wavefront of the data page had all pixels been in the ‘one’ state. Thus, successfully performing page-wide homodyne detection in such a manner may involve expensive, sophisticated adaptive optic elements and control algorithms in order to phase-match the local oscillator to the hologram (or vice versa). As such, performing homodyne detection is generally not practical in commercial holographic data storage systems.
Another approach to increasing the SNR of the reconstructed data page is quadrature homodyne detection as disclosed in U.S. Pat. No. 7,623,279, which is entitled “Method for holographic data retrieval by quadrature homodyne detection” and which has a filing date of Nov. 24, 2009. In quadrature homodyne detection, the reconstructed signal beam interferes with two versions of an imprecise local oscillator to produce a pair of interference patterns, e.g., one after another on the detector array. The two versions of the imprecise local oscillator are in quadrature, i.e., there is a 90-degree phase difference between them. As a result, the low-contrast areas in the interference pattern between the first version of imprecise local oscillator and the reconstructed signal beam appear as high-contrast areas in the interference pattern between the second version of imprecise local oscillator and the reconstructed signal beam. Similarly, the high-contrast areas in the interference pattern between the first version of imprecise local oscillator and the reconstructed signal beam appear as low-contrast areas in the interference pattern between the second version of imprecise local oscillator and the reconstructed signal beam. Combining the two interference patterns yields a completely high-contrast interference pattern that encodes all of the information in the reconstructed data page.